We are aware that when a metal bearing sulphide (e.g. a pyrite or arsenopyrite) is roasted, preferably after having been subjected to flotation, it is converted to a calcine. The calcine comprises mainly an iron oxide (ferric oxide) together with silica and some alkaline metal oxides with substantially no sulphide. The bulk of precious metal or any uranium which has been present in the sulphide can be recovered by leaching. However, some undissolved valuable metals are retained in the calcine. They may be retained due to being partially or completely encapsulated within iron oxide particles or by being in a solid solution within the crystals of iron oxide of the calcine. In either case, we are not aware of any practical commercial process for the recovery of such precious metals or uranium from the calcine. A complete breakdown of the iron oxide particle is first necessary before leaching, or the like can be carried out.
In mine dumps at Barberton in South Africa, the calcine waste from gold extraction can contain up to 20 grams of gold per ton of calcine as a presently unleachable refractory material. Other calcines may also contain uranium and other metals. It would be desirable to be able to extract these materials from the calcine.
We have now invented a process by which about 90% of the gold may be extracted and a high proportion of uranium.